Thermal Conductivity of Single Crystal FeSi

Recent research on bulk high quality single crystal FeSi found a surface conducting state the develops below 19 K with no change to FeSi’s bulk properties [1] – this state resembles a surface metallic state and is reminiscent of topological insulator behavior. Studies on thin film FeSi samples have also observed surface ferromagnetic domains [2], bolstering the evidence for a surface state in FeSi. By measuring the thermal conductivity of single crystal FeSi at low temperature, we can probe the correlation to the Wiedemann-Franz law and potentially separate the surface conducting state behavior from any bulk behavior. We performed thermal conductivity measurements on high quality FeSi single crystals with typical sample cross sections of 1600 um². Here we report experimental methods and results for thermal and electrical conductivity measured down to 1.8 K.